Telephonic switches provide an important part of the functionality of a modern telephone system. While in the early days of the telephone an operator manually completed signal connections, that function has now been largely assumed by the telephone switch.
Where a subscriber places a call, a switch controller within a local telephone switch first collects the dialed digits from a calling party and then analyzes the dialed digits to identify a called party. Where the leading digit is something other than a "1" and the number of digits totals seven, the switch controller knows the call is a local call and proceeds accordingly. The switch controller may begin by comparing the called number prefix (e.g., the first three digits) with a set of look-up tables to identify the telephone exchange serving the called party.
The switch controller will typically first compare the prefix of the number received from the calling party with the prefix of those called numbers served locally through the same switch. Where the called number is determined to be the same, the switch controller then goes on to identify the particular telephone connection serving the called party. Upon precisely identifying the called party, the switch controller sends a set of control instructions to the switch to connect the called party and calling party.
Where the switch controller determines that the called party is served through another switch, the switch controller must transfer a call request to a switch controller of the other switch. To do so the local switch controller first identifies (through look-up tables), a trunk group interconnecting the local switch with the other switch (either directly or through other intermediate switches), and transmits a call request through the identified trunk group. Upon receiving a response, the local controller commands the local switch to set up a call connection between the calling party and appropriate connection of the trunk group.
In some cases a calling party may be directing a call to an organization (e.g., a corporation, governmental agency, etc.), instead of a particular person. Often the organization may provide a single telephone number to be used in gaining access to the organization. Such calls, once identified, are often routed from the public switch telephone network (PSTN) into a private telephone system (private branch exchange (PBX)) of the organization through a special trunk group provided for that purpose.
Once within the PBX, the call may be routed to one of a group of people (agents) qualified to receive the call through an automatic call distribution (ACD) system. Often the ACD functions by simply looking for an open telephone line among the group of agents. In other cases, the ACD may use more sophisticated methods of routing calls to an agent of the group of agents, based upon qualification.
ACDs are also often capable of routing calls in an outbound direction during call campaigns from any one of a group of agents to any one of a group of customers of the organization. While the identity of the called party may be selected in sequence from a database of the organization, the assignment of an agent to the call may be based primarily on agent availability, or agent qualification.
In making the connection between a calling party and called party either from within the PSTN or a PBX, the switch must be capable of making an internal connection between any two external terminals. Often the solid state switching fabric must be integrated into a monolithic matrix which mist then be mated to the switching system as a whole.
Because of the need of universal internal connectivity, a telephone switch embodies a great deal of internal complexity. Because of the internal complexity, it is often not possible to expand the connection capabilities of telephone switches, once they are constructed and placed in operation. As a consequence, telephone switches are usually custom designed for an application, taking into account the number of connections needed and projected rate of growth of the market.
Where a switch is improperly sized, or growth exceeds expectations, a telephone switch may be rendered practically obsolescent long before technical obsolescence occurs. Where such an event occurs, the switch must be replaced at great expense.
Switches used in telecommunications are very expensive to build and install and there is a strong incentive to avoid oversizing such switches. Because the cost and lead times associated with telecommunications switches, a need exists for a practical means of expanding the switching capabilities of such devices after initial construction and installation.